This invention relates to the field of thermal energy storage systems and more particularly to a novel system for absorption and release of heat utilizing a condensed state phase change material.
Among the practical problems involved in solar energy systems is the need for an effective means by which the heat collected during periods of bright sunshine can be stored, preserved and later released for utilization during the night or other periods when the sun is not shining. Traditionally, energy collected during periods of sunshine has been stored in the form of sensible heat, typically by raising the temperature of rocks, water, oil, salt solutions, etc. Subsequent passage of a heat transfer fluid over the hot energy storage system releases the heat which then can be used to heat water, to provide residential heating, or for other uses.
In passive systems, an energy storage system serves as a buffer, absorbing energy from its environment when the surroundings are in the high temperature portion of a regular temperature cycle (as in daytime) and releasing energy to the surroundings during the low temperature portion of the cycle (as at night).
Storage of the thermal energy in the form of sensible heat creates practical problems in the design and operation of a solar heating or passive buffer system. Thus, to provide for storage of adequate quantities of heat energy in a solar system, it is necessary to either raise the thermal energy storage material to a high temperature or to utilize a large mass of heat storage material. Where high temperatures are used, solar collectors must operate at correspondingly high temperatures, which inhibits their efficiency and increases their complexity. The alternative of using a large mass of heat storage material necessitates a large space allocation and may also involve relatively expensive heat transfer apparatus.
In a passive system, the buffering capacity of a sensible heat storage material is generally very limited.
Thus, a need has existed for an improved thermal energy storage system which is relatively compact, has high buffering capacity for passive use, and can be operated at moderate temperature for collection and release of heat derived from solar energy or collected from other heat sources.
Among other heat sources of potential economic importance are industrial process streams which carry waste heat at relatively modest temperature. Such streams may not be hot enough for use in steam generation and, depending on the nature of the industrial process, there may be no practical opportunity to recover the heat in other conventional ways such as pre-heating of feed streams. With the rapid escalation of energy costs, a need has existed for improved techniques for the recovery of low temperature waste process heat.